An analytical expression for the vapor pressure of ionic liquids based on the Peng-Robinson equation of state is proposed. The method uses the concept of zero-pressure fugacity previously presented in the literature and that has been found to work well in the range of low vapor pressures (lower than 0.1 Pa), where the values of vapor pressure of ionic liquids, up to moderate temperatures, are found. The parameters of the equation of state are generalized in terms of ionic liquid properties and results are compared with experimental data, showing average absolute deviations below 8%. The advantage of the proposed analytical expression is its solid thermodynamic foundation, the generalization of the model parameters, and its acceptable accuracy for engineering calculations. (C) 2012 Elsevier B.V. All rights reserved.